diff --git a/src/backend/access/nbtree/nbtutils.c b/src/backend/access/nbtree/nbtutils.c
index aaa0c89c7dd..86cec248593 100644
--- a/src/backend/access/nbtree/nbtutils.c
+++ b/src/backend/access/nbtree/nbtutils.c
@@ -1725,6 +1725,7 @@ _bt_killitems(IndexScanDesc scan)
 	int			i;
 	int			numKilled = so->numKilled;
 	bool		killedsomething = false;
+	bool		droppedpin PG_USED_FOR_ASSERTS_ONLY;
 
 	Assert(BTScanPosIsValid(so->currPos));
 
@@ -1742,6 +1743,7 @@ _bt_killitems(IndexScanDesc scan)
 		 * re-use of any TID on the page, so there is no need to check the
 		 * LSN.
 		 */
+		droppedpin = false;
 		LockBuffer(so->currPos.buf, BT_READ);
 
 		page = BufferGetPage(so->currPos.buf);
@@ -1750,6 +1752,7 @@ _bt_killitems(IndexScanDesc scan)
 	{
 		Buffer		buf;
 
+		droppedpin = true;
 		/* Attempt to re-read the buffer, getting pin and lock. */
 		buf = _bt_getbuf(scan->indexRelation, so->currPos.currPage, BT_READ);
 
@@ -1795,9 +1798,18 @@ _bt_killitems(IndexScanDesc scan)
 				int			j;
 
 				/*
-				 * Note that we rely on the assumption that heap TIDs in the
-				 * scanpos items array are always in ascending heap TID order
-				 * within a posting list
+				 * We rely on the convention that heap TIDs in the scanpos
+				 * items array are stored in ascending heap TID order for a
+				 * group of TIDs that originally came from a posting list
+				 * tuple.  This convention even applies during backwards
+				 * scans, where returning the TIDs in descending order might
+				 * seem more natural.  This is about effectiveness, not
+				 * correctness.
+				 *
+				 * Note that the page may have been modified in almost any way
+				 * since we first read it (in the !droppedpin case), so it's
+				 * possible that this posting list tuple wasn't a posting list
+				 * tuple when we first encountered its heap TIDs.
 				 */
 				for (j = 0; j < nposting; j++)
 				{
@@ -1806,8 +1818,12 @@ _bt_killitems(IndexScanDesc scan)
 					if (!ItemPointerEquals(item, &kitem->heapTid))
 						break;	/* out of posting list loop */
 
-					/* kitem must have matching offnum when heap TIDs match */
-					Assert(kitem->indexOffset == offnum);
+					/*
+					 * kitem must have matching offnum when heap TIDs match,
+					 * though only in the common case where the page can't
+					 * have been concurrently modified
+					 */
+					Assert(kitem->indexOffset == offnum || !droppedpin);
 
 					/*
 					 * Read-ahead to later kitems here.